The fabrication of a single semiconductor device can require the careful synchronization and precisely measured delivery of as many as a dozen gases to a process chamber. Various recipes are used in the fabrication process, and many discrete processing steps where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., for example, may be required. The steps used, their particular sequence, and the materials involved, all contribute to the making of a semiconductor device.
Wafer fabrication facilities are commonly organized to include areas in which gas manufacturing processes, such as chemical vapor deposition, plasma deposition, plasma etching, and sputtering, are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition, are supplied with various process gases. The process gases are supplied to the tools in precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas box. The gas box delivers precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool. The gas box, or gas metering system, includes a plurality of gas paths having gas metering units, such as valves, pressure regulators and pressure transducers, mass flow controllers (MFC), and mass flow meters (MFM).
It is desirable and often times necessary to test, or verify, the accuracy of an MFC or an MFM. One way to verify an MFC or MFM is through a “rate-of-rise” (ROR) flow verifier. Referring to FIGS. 1 and 2, an exemplary embodiment of a ROR flow verifier 100 of the prior art is shown. In FIG. 1, the flow verifier 100 is shown connected between a gas manifold 122 and a vacuum pump 124 of a gas metering system 120. The gas metering system 120 also includes a plurality of MFCs 126 controlling the flow of gas through lines connected to the gas manifold 122, and the gas manifold 122 is connected to a process chamber 128. In the exemplary embodiment of FIG. 1, the system 120 includes four lines connected to the gas manifold 122 and having MFCs 126. The system, however, can include more or less than four lines, as desired. First and second on/off type valves 130, 132 alternately control flow from the manifold 122 to either the process chamber 128 or the flow verifier 100. Gate valves 134, 136 connect the vacuum pump 124 to the process chamber 128 and the flow verifier 100.
The flow verifier 100 is used to verify and, if desired, calibrate the rate of flow produced by the MFCs 126, either individually or in combination. As shown in FIG. 2, the apparatus 100 includes an inlet 114 connectable to a DUT, and an outlet 116 connectable to a vacuum pump for drawing gas through the DUT and the flow verifier. The verifier 100 also includes a vessel 102 having a predetermined volume, an “upstream” or first valve 104 controlling flow between the inlet 114 and the vessel 102, a “downstream” or second valve 106 controlling flow from the vessel 102 to the outlet 116, and a vessel pressure measurement device 108 communicating with the volume of the vessel 102. The flow verifier 100 may also include a bypass valve 110 controlling direct flow between the inlet 114 and the outlet 116 and in parallel with the valves 104, 106, the vessel 102 and the pressure transducer 108, as shown.
A computer controller 112 of the flow verifier 100 utilizes the ROR method of flow verification, which is illustrated in FIG. 3 by the graph of pressure (P) versus time (t). In general, the controller 112 is a computer processor that includes electronic memory and a clock. The controller 112 is generally programmed so that, during operation, the controller 112 first closes the bypass valve 110 and opens the first and the second valves 104, 106 so that flow is bypassed from the manifold 122 and through the vessel 102. The controller 112 is further programmed so that, after an initialization period to allow the bypassed flow to stabilize, the second valve 106 is closed to stop flow from the vessel 102. As the closed vessel 102 is filled with gas from the manifold, the controller 112 receives measurements of vessel pressure from the pressure measurement device 108, receives measurements of time from its clock, and determines a rate of change in vessel pressure due to the gas flow. The controller 112 then determines an actual flow provided by the MFC 126 connected to the manifold 122 using the rate of change in vessel pressure and the known volume of the vessel 102. The graph of FIG. 3 illustrates how the gas flow rate can be calculated by the controller 112 from the change in pressure over time (ΔP/Δt) in the known volume of the vessel 102.
After the flow measurement is made, the first valve 102, shown in FIG. 2, is closed and the second valve 106 is opened to purge the vessel 102 with the vacuum pump 124. After purging, the second valve 106 is closed and the bypass valve 110 is opened to allow normal flow between the manifold 122 and the vacuum pump 124.
The flow verifier 100 can comprise, for example, a GBROR® in-situ flow verifier or a Tru-Flo® in-situ flow verifier, both of which are provided by MKS Instruments of Wilmington, Mass. (http://www.mksinst.com). The GBROR® is a modular gas path, or stick, and includes the valves, the pressure vessel, the pressure transducer and the controller mounted on a manifold. The GBROR® and the Tru-Flo® flow verifiers are both process transparent, i.e., operate between the normal processing steps of the gas delivery system, and thus reduce processing tool down time. The pressure measurement device 108 can comprise, for example, a Baratron® brand pressure transducer, which is also available from MKS Instruments.
One problem associated with such ROR flow verifiers are measurement errors resulting from a connecting flow path volume (“external volume”) between the DUT and the ROR flow verifier, when the ROR flow verifier is not close-coupled to the DUT. An example of “external volume” is shown in FIG. 1 between the MFCs 126 and the flow verifier 100. When gas is flowing, there is a resulting pressure drop in the external volume upstream of the ROR flow verifier 100. This pressure drop causes flowing gas to have a greater density in the upstream external volume than in the volume-calibrated chamber 102 of the ROR flow verifier 100. The higher density in the upstream external volume causes an inaccurate pressure measurement in the volume-calibrated chamber 102 of the flow verifier 100, thereby causing ah error in the gas flow rate as detected by the flow verifier.
A time-consuming setup calibration is normally necessary to cope with large external volumes when using existing ROR flow verifiers where detailed and accurate information about the upstream plumbing and type are necessary. In addition, existing ROR flow verifiers require a vessel having a relative large volume (e.g., 200–2000 cc), which may prevent the flow verifiers from being installed in-situ in compact gas process delivery lines.
What is still desired is a new and improved flow verifier for verifying and, if desired, calibrating flow control devices in a gas metering system. Preferably, the new and improved flow verifier will employ the ROR method to verify flow. In addition, the new and improved flow verifier will preferably provide in-situ verification of flow control devices, so that the verification does not require removal of the flow control devices from the gas metering system. The new and improved, in-situ flow verifier will preferably be relatively small in size, and provides relatively fast and accurate flow measurements. Furthermore, the new and improved, in-situ flow verifier will preferably be substantially insensitive to the size of the external (connecting) volume located between the verifier and the device under test.